1. Field of the Invention
This invention relates to ceramic powders and compacts sintered therefrom; and more particularly to a lanthanum chromite ceramic powder and powder compact adapted to be sintered at low temperatures.
2. Description of the Prior Art
Solid oxide fuel cells (SOFC) have high Potential in producing electrical energy from cheap fuels or byproduct waste gas streams of the petrochemical and metallurgical industries. The potential of these fuel cells lies in the high efficiency of converting chemical to electrical energy and could find extensive applications in the domestic, commercial, defense, and aerospace sectors of the economy. The realization of this potential is contingent on the development of reliable and cost efficient methods of cell fabrication.
One of the solid oxide fuel cell designs resembles a heat exchanger with a honeycomb structure in which the electroactive ceramic components also serve as the structural members and eliminate the need for inert supports. This design is referred to as the monolithic solid oxide fuel cell (MSOFC). The honeycomb structure of the MSOFC is made up of thin layers of four components: (1) anode usually made of a nickel-zirconia cermet; (2) electrolyte made of a fully stabilized (cubic) zirconia; (3) cathode made of strontium-doped lanthanum manganite (LaMnO.sub.3); and, (4) interconnect made of doped lanthanum chromite (LaCrO.sub.3) The anode, electrolyte and cathode layers when co-sintered into a laminated structure, in the same order, make up one cell. The interconnect serves as an internal electrical connection between the individual cells and is also formed in a laminated structure sandwiched between anode and cathode layers. While the anode and cathode layers need to retain high porosity to facilitate gas-solid reactions, the electrolyte and interconnect layers must be sintered to closed porosity to prevent the intermixing of fuel and oxidant gases.
The monolithic solid oxide fuel cell offers lower material costs, the potential for reduced manufacturing costs, and a higher efficiency over other geometries and designs. However, fabrication of these cells is expected to be more complicated because the individual components in thin sheet form must be formed into multilayer sheets which are then converted into a honeycomb structure and must be co-sintered at the same relatively low temperature. Of particular importance is the sintering behavior of the interconnect material, that is, lanthanum chromite which must be sintered to close porosity or about 94% of its theoretical density at temperatures of about 1400.degree. C. in air or oxygen-containing atmospheres.
Lanthanum Chromite is a refractory material with a melting point of 2510.degree. C. which requires very high temperatures and controlled atmospheres, i.e. extremely low partial pressures of oxygen for sintering to near theoretical density. Groupp and Anderson (L. Groupp and H. U. Anderson), J. Am. Ceram. Soc., 59, 449 (1976)) have shown that LaCrO.sub.3 does not sinter in air even at temperatures as high as 1720.degree. C. According to the data reported by these investigators, LaCrO.sub.3 could be sintered to 95.3% TD only at 1740.degree. C. and in an atmosphere of nitrogen having an oxygen partial pressure of 10.sup.-11 atm. The main inhibition to densification appears to be the volatilization of chromium oxides in oxidizing atmospheres. The oxidation and volatilization of lanthanum chromite in oxidizing atmospheres at temperatures higher than 1400.degree. C. has indeed been reported by Meadowcroft and Wimmer (75th Annual Meeting of the Am. Ceram. Soc., Cincinnati, (1973) and D. B. Meadowcroft and J. M. Wimmer, Am. Ceram. Soc. Bull., 58, 610 (1979)) and involves the oxidation of Cr(III) to Cr(VI) and formation of fugitive CrO3 which is a gas at the high temperatures of sintering. Therefore, the preparation of lanthanum chromite powders which sinter to close porosity at temperatures below about 1450.degree. C., so that Cr volatilization is insignificant, is critical for the development of fuel cell fabrication technology. Reduction in the sintering temperature of a ceramic powder is achieved by controlling the composition, homogeneity, grain size, and morphology of the powder. The most promising approach in achieving all these requirements is to use sol- gel technology, i.e. solution chemistry, and improved powder separation and processing technology.
An improved sol-gel method has been disclosed by U.S. Pat. No. 4,830,780, to Olson et al., for the preparation of lanthanum chromite doped with the divalent ions of magnesium, strontium, calcium or barium by coprecipitation from salt solutions of lanthanum, chromium and dopant ions with ammonium hydroxide. In this patent disclosure, extensive washing of the precipitated gel is not needed because residual ammonium ion is removed via the gas phase during powder calcination. Upon calcination at temperatures of about 600.degree. C., the gel converts to a single compound with the huttonite structure, LaCrO4, which upon further calcination at 900.degree. C. converts to pure lanthanum chromite, LaCrO3, with average particle size of about 0.5 .mu.m. This powder could be sintered to 95.7% of theoretical density when fired at 1650.degree. C. for 4 hours in a graphite furnace and to 78 % theoretical density at 1600.degree. C. for 2 hours in a furnace with oxygen partial pressure of 10.sup.-10 atmospheres. Densification of this lanthanum chromite to the indicated densities was much better than what was achievable by the prior art as, for example, stated by Groupp and Anderson. However, the sintering temperature of this reactive powder is still higher than what is needed for monolithic solid oxide fuel cell applications.
Densification of lanthanum chromite at much lower temperatures has been disclosed in U.S. Pat. No. 4,749,632 to Flandermeyer et al. This was achieved by the incorporation into the lanthanum chromite of a sintering aid, that is, a compound or mixture of compounds which have melting points much lower than 1400.degree. C. For example, lanthanum chromite mixed with 10 w% boric acid powder was formed into a tape and fired at 1377.degree. C. to a density of about 94% of theoretical density. Note that boric acid, H.sub.3 BO.sub.3, melts at about 160.degree. C. with simultaneous dehydration to HBO.sub.2, while boron oxide, B.sub.2 O.sub.3, a product of boric acid upon further dehydration, melts at about 460.degree. C. In another example, the sintering aid was made up of 8 w% (Ca,Cr) oxide, which has a eutectic point at about 1022.degree. C., and 6 w% B.sub.2 O.sub.3 and, because of the low melting point of B.sub.2 O.sub.3, the melting point of this sintering aid mixture would be expected to be very much lower than 1000.degree. C. A mixture of lanthanum chromite with the [B.sub.2 O.sub.3, (Ca,Cr) oxide] sintering aid was fired to about 90% of theoretical density at 1277.degree. C.
Thus, U.S. Pat. No. 4,749,632 teaches sintering of lanthanum chromite at low temperatures by the incorporation of relatively large quantities of compounds which melt at low temperatures and are referred to as sintering aids. However, the use of relatively large quantities of low-temperature melting compounds would be expected to result in migration of some of the sintering aid ions into the adjacent layers during sintering and, therefore, affect the sintering behavior and electrical performance of these layers. These sintering aids may be deleterious to the fabrication and operational performance of the monolithic solid oxide fuel cell.